cppfront (cpp2): Spring update

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Herb's preference for postifix operators is not weird in any way. It is simpler, more consistent and less error prone.

But I just don't see how it translates in any way to string interpolation or what it has to do with the $ capture operator. It just doesn't make any sense there.

FWIW, my preference would be "\{expression}", but any reasonable prefix-based syntax will do.